Linearly polarized microwave feed assembly for parabolic antennas and the like

ABSTRACT

A first coaxial line, short-circuited at one end and propagating a TEM wave, is surrounded by a concentric sleeve of a length approximately equal to half the operating wave length, to provide a second line with the outer conductor of the first line acting as inner conductor of the second line. The latter has one shortcircuited end and an annular radiating aperture at its opposite end. The outer conductor of the first line has a pair of diametrically opposed coupling slots with the intervening segments connected, respectively, inductively and capacitatively with the inner conductor of said first line.

United States Patent lnventor Appl. No. Filed Patented Assignee PriorityAlfred Kach Untersiggenthal, Switzerland Nov. 29, 1968 May 25, 1971Patelhold Pa Holding AG tentverwertungs- & Elektro- Glarus, SwitzerlandDec. 1, 1967 Switzerland 16921 LINEARLY POLARIZED MICROWAVE FEEDASSEMBLY FOR PARABOLIC ANTENNAS AND [50] Field of Search 343/770,771,781, 818, 820, 822, 791, 840, 786

[5 6] References Cited UNITED STATES PATENTS 2,954,556 9/1960 Yang343/770X Primary Examiner-Herman Karl Saalbach Assistant ExaminerPaul L.Gensler Att0rneyGreene & Durr ABSTRACT: A first coaxial line,short-circuited at one end and propagating a TEM wave, is surrounded bya concentric sleeve of a length approximately equal to half theoperating wave length, to provide a second line with the outer conductorof the first line acting as inner conductor of the second line. Thelatter has one short-circuited end and an annular radiating aperture atits opposite end. The outer conductor of the first line has a pair ofdiametrically opposed coupling slots with the intervening segmentsconnected, respectively, inductively and capacitatively with the innerconductor of said first line.

LINEARLY POLARIZED MICROWAVE FEED ASSEMBLY FOR PARABOLIC ANTENNAS ANDTHE LIKE The present invention relates to a microwave feed assembly forparabolic antennas and the like designed for energization by a coaxialfeeder line and radiation of linearly polarized electromagnetic waves.

In the microwave art, dipole antennae are chiefly used as primaryradiators for the illumination of parabolic reflectors, among otherthings on account of the low gain factor and slight directional effectof the dipole radiator.

In a well-known dipole antenna of this type energized by a coaxial lineor feeder having two diametrically opposed longitudinal slots in itsouter conductor, a dipole stub is fitted to each of the segments thusformed of the outer conductor. In order to couple one of the dipolestubs to the inner conductor of the coaxial line, the conductor isconnected to the respective segment through a short-circuiting stub atright angles to and disposed at a point of the conductor coincident with.the bisecting plane of the slots. A splash or reflector disc arrangedon the side of the slots remote from the input side of the line insuresthat the parabolic reflector is optimally illuminated by the dipoleradiator. In the case of such an arrangement, a symmetrical coupling forthe dipole stubs is provided by the effect of the half-wave (M2)resonance of the two conductor segments.

Such an antenna with pronounced dipole stubs is especially suitable forfrequencies in the region of 1-4 gc. (gigacycles), corresponding to awave length range 30-7 cm. At higher frequencies, or for still shorterwave lengths, for example 6-8 gc. or about -3 cm., respectively, thedipole stubs become so small that, regard being had to the dimension ofthe feeder line, it is practically no longer possible to comply with theexisting geometrical and space requirements for the antenna to functionsatisfactorily. On the other hand, the use of dipole stubs hasheretofore been one of the most simple means for the illumination of aparabolic reflector with linearly polarized electromagnetic radiation.

An important object of the present invention is, therefore, theprovision of an improved dipole antenna of the referred to type which isespecially, though not limitatively, suitable as a primary radiator forthe illumination of a parabolic reflector with linearly polarized wavesby way of a coaxial feeder line, without requiring any pronounced dipolestubs, which anten na exhibits good radiation and matching propertiesover a relatively wide frequency range and in spite of its relativelylarge dimensions compared with the operating wave length; and which isboth simple in design and suitable for structural embodiment with aparabolic reflector.

The invention, both as to the foregoing and ancillary objects as well asnovel aspects thereof, will be better understood from the followingdetailed description of a preferred practical embodiment, taken inconjunction with the accompanying drawing forming part of thisspecification and in which;

FIG. 1 is a longitudinal sectional view of a microwave feed assembly forparabolic antennas for the radiation of linearly polarized waves andembodying the invention;

FIG. 2 is a sectional view taken on line 2-2 of FIG. I;

FIG. 3a and 311 show substitute electrical circuits explanatory of thefunction and operation of the invention; and

FIGS. 4a4c further illustrate the formation of a linearly polarizedradiation field in accordance with the principles of the invention.

Like reference characters denote like parts in the different views ofthe drawing.

With the foregoing objects in view, the dipole antenna according to theinvention is characterized generally by the connection of the innerconductor of a slotted coaxial feeder line to one of the segments of theouter conductor of said line by way of an inductive link or couplingimpedance, on the one hand, and by the connection of the inner conductorof the line to the other segment by way of capacitive link or couplingimpedance, respectively, the effective impedance values of thesecouplings being substantially equal and opposite at a mean operatingfrequency or wave length (A0) in the coupling plane which bisects theslots in the outer conductor of the line. Additionally, the inventioninvolves the provision of a further hollow outer conductor or concentricsleeve surrounding the coaxial feeder line at least over the region ofits longitudinal slots, whereby to provide a second line having an innerconductor formed by the outer conductor of the first line.

Due to the coupling of the second line by way of the inductive andcapacitive links between the inner conductor of the first line with thesegments of its outer conductor (forming the inner conductor of thesecond line), a TB wave is excited in the second line by a TEM wavepropagated in the first line energized by a suitable source of microwaveenergy. This TE wave exhibiting a substantial cross-field as a result ofthe geometry of the concentric conductors is converted into asubstantially linearly polarized field suitable for radiation by av TEMwave being directly coupled into the second line through the slots inthe outer conductor of the first line, the width of the slots being suchas to provide a TEM field in the second line substantially cancelling,at or near the radiation aperture of the line, the cross-field componentof the TE wave being radiated, in a manner as will become furtherapparent from the description of the construction and function of theinvention in reference to the drawing.

Referring to the latter, FIGS. 1 and 2 show a dipole antenna andparabolic reflector in longitudinal and transverse section,respectively. In the end portion 1 of a coaxial feeder line 2, energizedby a suitable microwave generator (not shown), there are provided twodiametrically opposite longitudinal slots 3 and 4 having a length equalto half the mean operating wave length (ho/2) and subdividing the outerconductor 5 of the line into two opposite segments 6 and 7,respectively. The inner conductor 8 of the line is linked or coupled viaa shorting stub 9 to the segment 6 at a point coincident with thecross-sectional plane x-x which bisects the slots 3 and 4 and which willhereafter be referred to as the coupling plane of the line. Ashort-circuiting disc 10, terminating the end portion 1 of the line 2 ata distance of five-eighthsseven-eighths of the slot length(five-sixteenthsseven-sixteenths of k0) from the coupling plane X-x,constitutes a composite structural part together with the innerconductor 8 and the shorting stub 9. Additional coupling elements in theform of stubs or screws 11 and 12 are fitted in the segments 6 and 7,respectively. The screw 11 moreover serves to fasten the stub 9 to thesegment 6.

An outer cylindrical conductor or sleeve 13 is secured, via a thread 14,to the outer conductor 5 of the line 2 and forms, together with saidconductor a further coaxial line or feeder of annular cross sectionwhich is terminated at one end, that is, adjacent to the outer ends I5of the slots in the example illustrated, by the base 16 of the sleeveacting as a short-circuiting disc. The opposite end of the sleeve 13 isextended by an insulating tube 17 terminating in a conical end piece17'. Line 2 may be fitted with a suitable impedance matching meanslocated ahead of the input-feed ends 19 of the slots 3 and 4 andconsisting, in the example shown of a step or shoulder 19' on the insideof the outer conductor 5 and Ito/4 line section between said shoulderand the ends 19 of slots 3 and 4.

The function and operation of the antenna described in the foregoingwill be explained with reference to FIGS. 3a, 3b and la-4c.

FIG. 3a shows an equivalent electric circuit diagram of the feederradiator according to FIGS. 1 and 2. It is assumed that the coaxialfeeder line 2 up to and including the AD/4 line sections has an internalresistance R The segments 6 and 7 of the outer conductor may then beconsidered as a two-wire transmission line 20 short-circuited at adistance of half a wave length from the input-feed plane and having itsindividual conductors connected to the inner conductor of the line atthe coupling plane x-x, or at a point one-quarter wave length from theinput-feed plane, via a coupling inductor L and a coupling capacitor C,respectively. The inductance L and capacitance C are so chosen thattheir impedance values are substantially equal and opposite at the meanoperating frequency of wave length (AOL-whereby to provide a symmetricalcoupling of the feeder line 2 with the antenna load or radiationresistance R,.

According to a first modification of the invention, the couplinginductance L and coupling capacitance C take the form, respectively, ofthe shorting stub and a trimmer screw 12 protruding into the coaxialfeeder and adjusted to provide equal and opposite inductive andcapacitive coupling .reactances between the inner conductor 8 andsegments 6 and 7 of the line 2. Such a solution, while suitable forcertain uses and applications is less advantageous than themodification,

. utilizing a fixed coupling screw 11 in conjunction with ashortcircuited end section of the conductor having a length equal tofive-sixteenthsseven-sixteenths A from the coupling plane Xx, in themanner as will become further apparent from the following description inreference to the modified circuit diagram of FIG. 3b.

Referring to the latter, the end portion 1 of the line 2, beingterminated by the short-circuiting disc at a distance offiveeights-seven-eigh ths of the slot length, orfive-sixteenthsseven-sixteenths of the operating wave length, from thecoupling plane x-x, is represented by an equivalent two-wire line 21being short-circuited at its end in the manner shown. The stub 9 whichconnects the inner conductor 8 to the segment 6 is represented in theequivalent circuit by the inductor L. As may easily be seen, the endportion 1 of the feeder line 2, being terminated at the stated distancefrom the coupling plane x-x, provides a capacitive coupling for thesegment 7 corresponding to the capacity C according to FIG. 30, on theone hand, while additionally acting to transform the inductance L of thestub 9 up to approximately twice of the inductance L according to FIG.3a, on the other hand.

Thus with, the same stub 9, the arrangement as shown in FIG. 1 providesa doubling of the effective coupling inductance compared with thepreviously mentioned variant of a capacitive coupling by means of atrimmer stub or screw 12, thus providing a four times impedancetransformation for matching the feeder line to the load resistance R ofthe antenna. As a consequence, the cross section of the stub 9 may bemade large enough, even at very high operating frequencies, to impartgood mechanical stability to the arrangement when usingimpedance-transformation ratios as required in practice.

The symmetrical coupling of the segments 6 and 7 results in theexcitation of an electric TE wave in the annular space or outer lineenclosed by the sleeve 13 and conductor 5, the field of said wave in thecoupling plane x-x being illustrated in FIG. 40. Due to the circularcylindrical surfaces or geometry of the electrodes, this field has arelatively strong transverse or crosscomponent, greatly distortingthereby the ideal linearly polarized field. Besides the TE wave, astanding TEM wave, whereof the field in the coupling plane x-x is shownin FIG. 4b, is excited via and in the region of the slots 3 and 4, inthe outer wave guide or line between conductors 5 and 13.

Inasmuch as the cross-components of the TE field and of the TEM fieldare in the same direction in the coupling plane x-x, superimposition ofthe fields at this point further increases the distortion of thelinearly polarized wave set up in the outer line. However, since thecross-sectional dimensions of the outer line are so chosen that, at theoperating frequencies involved, only the TE wave is excited in the outerline, while the field of the standing TEM wave penetrates into said lineonly in heavily attenuated form, optimal compensation of the interferingcross-components of the TE field may be achieved by rotation of thephase of the TE wave by 180. It is for this purpose that the length ofthe sleeve 13 is so chosen that the distance of its open end orradiating aperture from the coupling plane x-x is at least substantiallyequal to half the operating wave length of the TE, wave. Optimalcompensation of the cross-components of the TE field may thus beachieved at or near the radiating aperture or open end of the outer lineor wave guide. The strength of the field of the TEM wave can moreover beso adapted, such as by varying the width of the coupling slots 3 and 4,that the crossfield components of the TE wave are substantiallycompletely compensated at the open end or radiating aperture of theantenna.

The resultant field of the TE wave with its distortion removed in thismanner at the open end or radiating aperture of the outer line is shownin FIG. 4c, illustrating the attainment of substantially symmetricallinearly polarized waves radiated from the open end of the outer lineand in turn by the antenna 22. The insulating tube 17 which extends theouter line from its open end serves to partially delay the radiatedwaves, whereby as the electromagnetic field emerges from the open end ofthe insulating tube, the electric field lines extend at least in parttangentially to the conical surface of the end piece 17. Experimentshave shown that it is advantageous for the length of the cylindricalpart of the insulating tube 17 to be approximately half the length ofthe radiated TE wave, with the tube extending over about half its lengthinto the hollow conductor or sleeve 13.

When the antenna is used as a rear feed radiator of a parabolicreflector, as shown at 22, FIG. 1, the insulating tube 17 impartsincreased divergence to the radiated waves, resulting thereby in a moreuniform illumination of the parabolic reflector. The optimum cone angledepends on the irradiation angle which it is required to attain. Goodresults have been achieved with a cone angle of 30 for most practicalpurposes. The insulating tube 17 furthermore acts at least partly as atransformation means between the wave-resistance of free space and theradiation resistance (R of the line 5, l3, and also protects the antennafrom the effects of atmospheric infl uence.

The screws 11 and 12 furthermore serve as coupling stubs for the purposeof matching the line 2 to the symmetrizing transformer formed by themeans for the symmetrical coupling of the segments 6 and 7 to the innerconductor 8 of line 2.

In the foregoing, the invention has been described in reference to anexemplary illustrative device. It will be evident that variations andmodifications, as well as the substitution of equivaLent parts ordevices for those shown for illustration, may be made in accordance withthe broader scope and preview of the invention.

Iclaim;

l. A linearly polarized microwave feed assembly for parabolic antennasand the like comprising in combination:

I. a first coaxial feeder line having an inner conductor and an outerconductor,

2. said outer conductor being provided with a pair of opposedlongitudinal slots located adjacent to the output end of said feederline and having a length approximately equal to one-half the operatingwave length, to provide a pair of intervening segments of said outerconductor between said slots,

3. means to short circuit the output end of said line,

4. a concentric metallic sleeve surrounding said feeder line inoverlying relation to said slots, to provide a second coaxial feederline with the outer conductor of said first feeder line forming theinner conductor of said second feeder line,

5. means to short circuit one end of said second feeder line, to providea radiating aperture at the opposite end of said second feeder line, and

6. a pair of coupling means to inductively couple the inner conductor ofsaid first feeder line with one of said segments and to capacitivelycouple said inner conductor of said first feeder line with the other ofsaid segments of said outer conductor of said first feeder line,

7. said inductive and capacitive coupling means offering substantiallyequal and opposite impedances, to excite a symmetrical TE wave in saidsecond feeder line by a TEM wave propagated through said first feederline, and

8. the length of said sleeve from the bisecting plane of said slots tosaid aperture being about one half of the operating wave length and saidslots having a width sufficient to apply an additional standing TEM waveto said second feeder line having a cross field component substantiallycancelling the cross field component of said TE, wave at said aperture.

2. In a microwave feed assembly as claimed in claim 1, wherein saidinductive coupling means consists of a short-circuiting stub coincidentwith said bisecting plane and connecting said inner conductor to one ofsaid segments, and wherein said capacitive coupling means consists of anadjustable coupling stub mounted in the other of said segmentscoincident with said plane.

3. in a microwave feed assembly as claimed in claim 1, wherein saidinductive coupling means consists of a short-circuiting stub disposedwithin said bisecting plane and connecting said inner conductor to oneof said segments, and wherein said capacitive coupling means consists inthe short-circuited end of said first line having a spacing distancefrom said plane equal to five-sixteenthsseven-sixtecnths of theoperating wave length. i

4. In a microwave feed assembly as claimed in claim 1, including aninsulating tube coaxial with and extending from the aperture ofsaidsleeve.

5. In a microwave feed assembly as claimed in claim 1, wherein theshort-circuiting means of said second line is disposed on the sideadjoining the short-circuiting means of said first line, and a parabolicreflector traversed by said first line and disposed in spaced relationto said aperture.

6. In a microwave feed assembly as claimed in claim 1, wherein theshort-circuiting means of said second line is disposed on the sideadjoining the short-circuiting means of said first line, an insulatingtube coaxial with and extending from the aperture of said sleeve, saidtube terminating in a conical-shaped and piece, and a parabolicreflector traversed by said first line and disposed in spaced relationto said end piece.

1. A linearly polarized microwave feed assembly for parabolic antennasand the like comprising in combination:
 1. a first coaxial feeder linehaving an inner conductor and an outer conductor,
 2. said outerconductor being provided with a pair of opposed longitudinal slotslocated adjacent to the output end of said feeder line and having alength approximately equal to one-half the operating wave length, toprovide a pair of intervening segments of said outer conductor betweensaid slots,
 3. means to short circuit the output end of said line,
 4. aconcentric metallic sleeve surrounding said feeder line in overlyingrelation to said slots, to provide a second coaxial feeder line with theouter conductor of said first feeder line forming the inner conductor ofsaid second feeder line,
 5. means to short circuit one end of saidsecond feeder line, to provide a radiating aperture at the opposite endof said second feeder line, and
 6. a pair of coupling means toinductively couple the inner conductor of said first feeder line withone of said segments and to capacitively couple said inner conductor ofsaid first feeder line with the other of said segments of said outerconductor of said first feeder line,
 7. said inductive and capacitivecoupling means offering substantially equal and opposite impedances, toexcite a symmetrical TE11 wave in said second feeder line by a TEM wavepropagated through said first feeder line, and
 8. the length of saidsleeve from the bisecting plane of said slots to said aperture beingabout one half of the operating wave length and said slots having awidth sufficient to apply an additional standing TEM wave to said secondfeeder line having a cross field component substantially cancelling thecross field component of said TE11 wave at said aperture.
 2. said outerconductor being provided with a pair of opposed longitudinal slotslocated adjacent to the output end of said feeder line and having alength approximately equal to one-half the operating wave length, toprovide a pair of intervening segments of said outer conductor betweensaid slots,
 2. In a microwave feed assembly as claimed in claim 1,wherein said inductive coupling means consists of a short-circuitingstub coincident with said bisecting plane and connecting said innerconductor to one of said segments, and wherein said capacitive couplingmeans consists of an adjustable coupling stub mounted in the other ofsaid segments coincident with said plane.
 3. In a microwave feedassembly as claimed in claim 1, wherein said inductive coupling meansconsists of a short-circuiting stub disposed within said bisecting planeand connecting said inner conductor to one of said segments, and whereinsaid capacitive coupling means consists in the short-circuited end ofsaid first line having a spacing distance from said plane equal tofive-sixteenths- seven-sixteenths of the operating wave length.
 3. meansto short circuit the output end of said line,
 4. a concentric metallicsleeve surrounding said feeder line in overlying relation to said slots,to provide a second coaxial feeder line with the outer conductor of saidfirst feeder line forming the inner conductor of said second feederline,
 4. In a microwave feed assembly as claimed in claim 1, includingan insulating tube coaxial with and extending from the aperture of saidsleeve.
 5. In a microwave feed assembly as claimed in claim 1, whereinthe short-circuiting means of said second line is disposed on the sideadjoining the short-circuiting means of said first line, and a parabolicreflector traversed by said first line and disposed in spaced relationto said aperture.
 5. means to short circuit one end of said secondfeeder line, to provide a radiating aperture at the opposite end of saidsecond feeder line, and
 6. a pair of coupling means to inductivelycouple the inner conductor of said first feeder line with one of saidsegments and to capacitively couple said inner conductor of said firstfeeder line with the other of said segments of said outer conductor ofsaid first feeder line,
 6. In a microwave feed assembly as claimed inclaim 1, wherein the short-circuiting means of said second line isdisposed on the side adjoining the short-circuiting means of said firstline, an insulating tube coaxial with and extending from the aperture ofsaid sleeve, said tube terminating in a conical-shaped and piece, and aparabolic reflector traversed by said first line and disposed in spacedrelation to said end piece.
 7. said inductive and capacitive couplingmeans offering substantially equal and opposite impedances, to excite asymmetrical TE11 wave in said second feeder line by a TEM wavepropagated through said first feeder line, and
 8. the length of saidsleeve from the bisecting plane of said slots to said aperture beingabout one half of the operating wave length and said slots having awidth sufficient to apply an additional standing TEM wave to said secondfeeder line having a cross field component substantially cancelling thecross field component of said TE11 wave at said aperture.